Field of the Invention
The invention relates, in general, to a device for suppressing undesired ignitions in a spark ignition engine. Such a device is known from EP 0 370 301 A1.
If an ignition coil is energized by an ignition output stage when a spark ignition engine is being controlled, the switch-on signal edge induces a voltage on the secondary side of the ignition coil when the primary circuit is switched on. This voltage is capable of generating an ignition spark at the spark plug given specific operating states of the spark ignition engine. Such an ignition spark can lead to undesired ignition of the petrol/air mixture, and can damage the engine because of an excessively early ignition angle.
An ignition coil is a system that is capable of oscillating and that is made of inductors and parasitic capacitors. If the system has a switch-on signal edge applied to it, among other things, the secondary winding is excited so that it undergoes damped oscillation, which can reach a voltage amplitude that is virtually twice as high as the voltage determined by the transformer property of the ignition coil.
For this reason, efforts are made to slow down the switching-on process of the ignition coil in such a way that no oscillation is excited, and the secondary voltage thus assumes only the voltage value that is determined by the transformer property of the ignition coil.
In the circuit that is known from EP 0 370 301 A1, a passive damping network is used that is configured between the positive pole and the negative pole of the voltage source of a vehicle's electrical system. The passive damping network is made of a series circuit including two resistors and a capacitor and has an output that is connected to the connecting point between the ignition switch and primary winding of the ignition coil. The fact that the voltage at the capacitor that discharges via the ignition switch decreases when the ignition switch is switched on slows down the voltage drop at the ignition switch. For this purpose, a large capacitance value of the capacitor is already necessary for the anticipated current limiting of the primary current. The capacitor without such current limiting would have to assume unrealistically high values in order to be able to cause the switching-on signal edge of the primary current to slow down.
Further circuits of this type can be found in DE 29 27 058 A1 and DE 196 12 984 A1.